Ultrasonic imaging has become a widely used tool in medical applications. Ultrasound techniques introduce high-frequency acoustic waves into a subject's body. The received echoes of those waves provide information allowing a trained observer to view the subject's internal organs. Ultrasound imaging equipment uses transducers that convert electrical energy into acoustic energy. Piezo-electric crystals are one commonly used type of electrical to acoustical transducer. To obtain a clear image, a high signal to noise ratio is desirable to overcome random noise associated with the imaging process. One way to increase the signal-to-noise ratio is to increase the amplitude of the signal driving the transducer. Generally, the transducer drive signal may require voltages in the range of +/−75 volts to +/−100 volts.
There are two broad categories of ultrasound transmitters, digital and analog. The analog type takes a signal generated digitally and after being converted to analog form, by a digital to analog converter, the signal is amplified to the required higher voltage by a power amplifier. This type of transmitter is capable of generating complex waveforms by using a high-resolution digital to analog converter with a resolution of, for example, 12 bits. This technique is expensive and finds application in high-end ultrasound imaging systems.
Digital transmitters are simpler and less expensive than analog transmitters. Unfortunately, the semiconductor process technologies used to fabricate digital circuits do not typically accommodate the high voltages required to produce an acceptable signal-to-noise ratio in an ultrasound imager. Moreover, lower voltage processes are often faster and less expensive. Thus, an ultrasound transmitter compatible with low-voltage semiconductor processes is desirable.